ADSL (Asymmetric Digital Subscriber Line) is a technology that uses the ordinary copper phone line for high-speed Internet connection. ADSL is currently undergoing deployment in North America, parts of Europe, as well as parts of China.
The Discrete Multitone (DMT) modulation is the choice for ADSL. As a standard, DMT is adopted internationally by American National Standards Institute (ANSI), European Telecommunications Standards Institute (ETSI) and the International Telecommunications Union (ITU). There is a complete specification and there are numerous independent manufacturers developing DMT technology.
DMT's ANSI T1.413 standard specifies 256 sub-channels, each with a 4 kHz bandwidth. They can be independently modulated from zero to a maximum of 15 bits/sec/Hz. This allows up to 60 kbps per sub-channel. At low frequencies, where copper wire attenuation is low and signal to noise ratio (SNR) is good, it's common to use a very dense constellation—greater than 10 bits/Hz is typical. In unfavorable line conditions, modulation can be relaxed to accommodate lower SNR—usually 4 bits/Hz or less.
In July 2002, the ITU completed G.992.3 and G.992.4 as new standards for ADSL called ADSL2. ADSL2 has been designed to improve the rate and reach of ADSL. On long telephone lines ADSL2 provide a data rate increase of 50 kbps. This results in an increase in reach of about 600 feet (with original data rate). At the loop reach as 20 kilo feet (about 6 km), the data rate increases from original ADSL's 100 kbps to the new ADSL2's 150 kbps.